Electrical Engineering presentation for Underground Cables.pptx

#Underground Cables
#Comparison between Underground Cables and Overhead Lines
# Resistance, Capacitance and Dielectric Loss Angle of Cables

1. Underground Cables Underground cables are also used for carrying the power from one end to another end. Though the installation cost of these cables is high, they are more reliable and safe. Sometimes it is not practical to use transmission lines, and it becomes necessary to use Cables.

2. An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover The main parts of underground cables are: Core or Conductor Insulation Metallic sheath Bedding Armouring Serving

3. (1) Cores or Conductors: The conductor of cable could be of aluminum or copper, Cable may have one or more than one core depending upon the type of services for application. It may be: Single Core, Two Core, Three Core or Four Core.

4. (2) Insulation: Each core or conductor is provided with a suitable thickness of insulation. The thickness of insulation layer depending upon the voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral.

5. (3) Metallic sheath: In order to protect the cable from moisture, gases or other damaging liquids in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation as shown in Fig.

6. (4) Bedding: Over the metallic sheath is applied a layer of bedding which consists of a fibrous material , the purpose of bedding is to protect the metallic sheath against corrosion and from mechanical damage.

7. (5) Armouring: Over the bedding, armouring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling. Armouring may not be done in the case of some cables.

8. (6) Serving: In order to protect armouring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armouring.

9. Classification of underground cables The classification of Underground cables can be done on the basis of several criteria, such as: Number of conductors in the cable Voltage rating of the cable Construction of cable Type and thickness of insulation used Installation and Laying of the cables

10. Classification Based Upon Number Of Conductors In The Cable Single core cable Three core cable Typically, an Underground cable has either one, three or four cores. Underground cables are usually employed to deliver 3 phase power. A 3 cored cable is preferred up to 66 kV. Beyond that, insulation required for the cable is too much. For higher voltages, 3 cored constructions become too bulky, and hence, even with some limitations we employ single cored cables

11. (A) Classification as per Voltage Capacity LT Cables :Low tension cables with maximum capacity of 1000 V. HT Cables :High tension cables with maximum capacity of 11KV. ST Cables :Super tension cables with rating capacity of 22KV- 33KV. EHT Cables :Extra High tension cables with rating capacity of 33KV- 66KV. Extra Super voltage cables: with maximum voltage rating beyond 132KV.

12. Classification as per construction of cable Belted Cables : Maximum voltage of 11KV. Screened Cables : Maximum voltage of 66KV. Pressure Cables : Maximum voltage of more than 66KV https://www.electricaleasy.com/2017/03/types-of-underground-cables.html

13. Classification Based Upon Insulation Of The Cable Various insulating materials used in cable construction are Rubber, Paper, PVC, XLPE (Cross linked Polyethene) etc. Such classification is based upon operating temperature limitations.

14. Insulation material Max. operating temperature PVC TYPE A 75°C PVC TYPE B 85°C PVC TYPE C 85°C XLPE 90°C RUBBER 90°C RUBBER – EPR IE-2, EPR IE-3, EPR IE-4, SILICON IE-5 150°C Following are some insulating materials used and their maximum operating temperatures

15. Classification Based Upon Installation And Laying Of The Cable Direct Buried: As the name suggests, the conductors are buried underground in a trench without additional accessories. Sometimes cooling pipes are added if required. Once the cables are installed, there’s no visible sign above the ground. Trough: Concrete troughs are dug and cables are installed in them. They’re visible on the surface. Maintenance is easier. https://uomustansiriyah.edu.iq/media/lectures/5/5_2020_04_04!07_49_13_PM.pdf

16. Tunnels: Sometimes, tunnels are dug up for this purpose. Such construction is mainly employed if a river needs to be crossed or if the intended power distribution is to a major city. Maintenance and future expansion is easier, but initial cost is higher. Gas Insulated Lines: This is a relatively new technology. For cables operating at higher voltages and currents, and handling high power, such gas insulated line construction is safer. It is being employed nowadays for advanced projects.

17. A comparison between underground cables and overhead T.L. 1. Construction Underground cables are more expensive , Construction of the cables is more complicated compared to the overhead T.L. which are simple to construct T.L.s do not require sheathing and other protective covers and are cheaper to construct.

18. 2. Size of Conductors Underground cables have larger conductor sizes compared to overhead lines for the same amount of power. This is due to the fact that the overhead lines have a natural cooling and hence the ability to carry more power without heating up.

19. 3. Voltage Ratings The overhead lines are better suited to carry higher voltages compared to the underground cables, which are limited by the expensive construction and limited heat dissipation. For these reasons, the underground cables are mostly used for transmitting up to 33KV

20. 4. Fault detection and repair It is easier to detect and repair faults in overhead cables. It is more complicated and takes more time to locate and repair the underground systems.

21. 5. Public safety Underground cables are safer to the public, animals and environment compared to the overhead lines i.e. there are no issues such as people getting in contact with fallen lines.

22. 6. Interference Overhead lines interfere with communication lines that are in close proximity, Have corona discharge, radio and TV interference which does not happen with the underground cables.

23. 7. Voltage drop There is more voltage drop in the overheads lines due to the fact that their conductors are of much smaller diameter than underground cables for the same power delivery.

24. Insulation Resistance of a Single-Core Cable The cable conductor is provided with a suitable thickness of insulating material in order to prevent leakage current. The path for leakage current is radial through the insulation. The opposition offered by insulation to leakage current is known as insulation resistance of the cable. For satisfactory operation, the insulation resistance of the cable should be very high.

25. Consider a single-core cable of conductor radius r1 and internal sheath radius r2 as shown in Fig. Let L be the length of the cable and ρ be the resistivity of the insulation. Consider a very small layer of insulation of thickness dx at a radius x. The length through which leakage current tends to flow is dx and the area of X-section offered to this flow is 2π x L.

26. Insulation Resistance (IR) of the assumed layer = ρdx/ 2πxL (From Resistance = ρL/A) Now, for getting the IR of whole cable we need to integrate up to a radius of R2 starting from R1 and to a length L. Insulation Resistance, R Thus it is clear from the above expression that Insulation resistance is Inversely Proportional to the length of Cable.

27. Capacitance of a Single-Core Cable A single-core cable can be considered to be equivalent to two long co-axial cylinders. The conductor (or core) of the cable is the inner cylinder while the outer cylinder is represented by lead sheath which is at earth potential. Consider a single core cable with conductor diameter d and inner sheath diameter D.

28. Let the charge per meter axial length of the cable be Q coulombs and ε be the permittivity of the insulation material between core and lead sheath. Obviously ε = ε0 εr where εr is the relative permittivity of the insulation. Where ε0 = 8.84X10 −12 𝐹/𝑚 Consider a cylinder of radius x meters and axial length 1 meter.

29. The surface area of this cylinder is = 2 π x × 1 = 2 π x m 2 Hence, Electric flux density at any point P on the considered cylinder is

30. Electric intensity at point P,

31. The work done in moving a unit positive charge from point P through a distance dx in the direc­tion of electric field is E x dx. Hence, the work done in moving a unit positive charge from conductor to sheath, which is the potential difference V between conductor and sheath, is given by :

32. Capacitance of the cable is

33. If the cable has a length of l meters, then capacitance of the cable is

34. Dielectric Stress in a Single Core Cable Under operating conditions, the insulation of a cable is subjected to elec­trostatic forces. This is known as dielectric stress. The dielectric stress at any point in a cable is infact the potential gradient (or electric inten­sity) at that point.

35. Consider a Capacitance of Single Core Cable with core diameter d and internal sheath diameter D. As already proved, the electric intensity at a point x metres from the centre of the cable is

36. By definition, electric intensity is equal to potential gradient. There­fore, potential gradient g at a point x meters from the centre of cable is

37. As already proved, potential difference V between conductor and sheath is

38. Substituting the value of Q from exp. (ii) in exp. (i), we get,

39. It is clear from exp. (iii) that potential gradient varies inversely as the distance x. Therefore, potential gradient will be maximum when x is minimum i.e., when x = d/2 or at the surface of the conductor. On the other hand, potential gradient will be minimum at x = D/2 or at sheath surface. Maximum potential gradient is

40. Minimum potential gradient is

41. The variation of stress in the dielectric is shown in Fig. 11.14. It is clear that dielectric stress is maximum at the conductor surface and its value goes on decreasing as we move away from the conductor. It may be noted that maximum stress is an important consideration in the design of a cable. For instance, if a cable is to be operated at such a voltage that maximum stress is 5 kV/mm, then the insulation used must have a dielectric strength of at least 5 kV/mm, otherwise breakdown of the cable will become inevitable.

42. Capacitance of Three Core Cables In three core cables, capacitances exist between the cores and between each core and the sheath. These capacitances are significant as the dielectric constant of the insulation (dielectric material) in cables is much more than the air. The capacitances are shown in the Fig. 1.

43. The core to core capacitances are denoted as C c while core to sheath capacitance are denoted as C s . The core to core capacitances C c are in delta and can be represented in the equivalent star as shown in the Fig. 2.

44. The impedance between core 1 and the star point, Z1 can be obtained as,

45. If star point is assumed to be at earth potential and if sheath is also earthed then the capacitance of each conductor to neutral is,

46. If Vph is the phase voltage then charging current per phase is

47. Measurement of C s and C c The practical measurement of C s and C c involves two cases : Case 1 : The core 2 and 3 are connected to sheath. Thus the C c between cores 2 and 3 and C s between cores 2, 3 and sheath get eliminated as shown in the Fig.

48. All the three capacitances are now in parallel across core 1 and the sheath. The capacitance of core 1 with sheath is measured practically and denoted by Ca. Ca = Cs + 2Cc ............(1) Case 2 : All the three cores are joined together. This eliminates all the core-core capacitances. This is shown in the Fig. 5.

49. The capacitances Cs are in parallel between the common core and sheath. This capacitance is practically measured and denoted as Cb. Cb = 3 Cs ...........(2) Solving (1) and (2) simultaneously, Ca = (Cb /3) + 2Cc Cc = (Ca /2)-(Cb /2) and Cs = Cb /3 Thus both the capacitances can be determined. CN = Cs + 3Cc =(Cb/3) + 3((Ca /2) -(Cb /2))

50. Current Carrying Capacity of Underground Cables The safe current-carrying capacity of an underground cable is determined by the maximum permissible temperature rise. The cause of temperature rise is the losses that occur in a cable which appear as heat. These losses are Copper losses in the conductors Hysteresis losses in the dielectric Eddy current losses in the sheath

51. The safe working conductor temperature is 65°C for armoured cables and 50°C for lead-sheathed cables laid in ducts. The maximum steady temperature conditions prevail when the heat generated in the cable is equal to the heat dissipated. The heat dissipation of the conductor losses is by conduction through the insulation to the sheath from which the total losses (including dielectric and sheath losses) may be conducted to the earth. Therefore, in order to find permissible current loading, the thermal resistivities of the insulation, the protective covering and the soil must be known.